BY .TAS. P. HILL. 75 



The very fact of the constant occurrence in Perameles of this 

 mode of birth by a direct median passage, even formed as it is in 

 by far the greater part of its exten t by rupture of maternal tissue 

 involving the loss of more or less blood at each act of parturition, 

 shows that with all its apparent defects it has proved of such 

 direct advantage as to have led to its adoption in preference to 

 the route offered by the lateral vaginal canals. What this 

 advantage is, is not far to seek when we contrast the two routes. 

 By way of the median passage, the young reach the exterior by 

 the shortest possible path; they simply jsass back in a straight 

 line, while to reach the exterior through the lateral vaginae they 

 must first pass back into one of the median vaginse, then directly 

 forwards through the anterior portion of one of the lateral canals 

 into the corresponding vaginal ctecum and hence back again 

 through the posterior portion of the lateral canal to the uro- 

 genital sinus. Parturition then through this latter path must, 

 we can easily imagine, have been not only a slow and laborious 

 process, but one difficult of successful accomplishment and even 

 fraught with danger to the lives of the young, cumbered as they 

 are with attached allantoic stalks At all events, the acquirement 

 of an entirely new passage is quite sufficient to show that the old 

 route proved in some way to be unsatisfactory. 



Now the origin of this new and direct passage in the first 

 instance presupposes, it seems to me, the existence of the median 

 vaginal cul-de-sacs. These may have originally arisen as out- 

 bulgings mechanically produced by the young to facilitate their 

 passage from the contracted neck of the uterus into the lateral 

 vaginal canal, here bent outwards and forwards in association 

 with the mesial position of the ureter. Whether or not this be 

 the true explanation of the origin of the vaginal cul-de-sacs, if 

 we grant their existence, then it seems probable that the median 

 passage was discovered through what we can only describe as an 

 accident, which, happening again and again, came eventually, 

 owing to its value, to be adopted as a normal occurrence. 



In the lowly Perameles, the old accidentally discovered passage 

 has persisted, probably unmodified, in correlation with the reten- 



